The present invention relates to an improvement in devices for non-contact gauging of thickness or weight per unit area of sheet or like materials using radioactive sources, and can be utilized for controlling the production of paper, textiles, films and like materials.
It is to be understood that the sheet materials mentioned herein include not only sheets as such from various materials both flexible and rigid, but also films, strips, as well as hollow articles, wherein the section of a wall being gauged can be considered as a section of a flat body such as a sheet.
Known in the prior art are various devices for noncontact gauging of thickness or weight per unit area of sheet or like materials, also referred to as thickness gauges and weight per unit area gauges. Such devices operate on the principle of measuring the degree to which charged ionizing radiation is absorbed by the material being gauged. They comprises a source of radiation and a detector, placed on opposite sides of the sheet material, and a special scanning means is provided for a synchronous movement of the source of radiation and the detector longitudinally or transversely over the surface of the material. There is a stringent requirement imposed upon such devices that the source of radiation, the detector and the material be kept in an invariably spaced, i.e., fixed relative relationship, because in gauging thin materials the mass of air in the measuring gap is comparable with the mass of the sheet, and any change in the gap, i.e., any displacement of the source relative to the detector in the plane of gauging will cause an inevitable error of gauging. Fulfillment of the requirement results in a more complicated scanning means which involves difficulties in making, mounting and operating the same, thus considerably raising the cost of the entire device.
In gauging sheet materials differing in thickness sources of radiation of different activity and energy have to be used. To provide for sufficient sensitivity of the devices in gauging thin sheets a source of soft (low-energy) radiation is used, whereas in gauging thick plates, sources of hard (high-energy) radiation should be applied.
There are also prior art devices whose operation depends on measuring ionizing radiation back-scattered from the material being gauged. In such devices the radioactive source of radiation and the detector are positioned on one side of the material being gauged, the stability of their arrangement in space being ensured by the rigidity of the detector construction. Though such devices have comparatively simple structural means for synchronous displacement of the source of radiation and the detector, nevertheless they suffer from a number of disadvantages.
Low intensity of back-scattered radiation necessitates the use of highly sensitive detectors which are of low stability and of limited service life.
Additionally, such devices should meet stringent requirements that there be a constant gap between the material and the detector, since a change in the gap will lead to an apparent change in the parameter being measured due to the shift of a portion of the back-scattered radiation beyond the detection zone.
Results of gauging also depend on the composition of the material being gauged.
Known in the art is a device for non-contact gauging of thickness or weight per unit area of sheet and like materials, as disclosed in British Pat. No. 1,338,157.
The above device comprises a source of radiation adapted to produce ionizing radiation and located on one side of the material, a magnetic means for arcuately deflecting the radiation that has passed through the material to enable it to pass through the material in the reverse direction, and a radiation detector to perceive the radiation that has passed through the material and located on the same side of the material as the source of radiation.
This device operates on the principle of measuring the magnitude of double adsorption of the radiation by the material, the detector and the radiation source being arranged on one side of the material being gauged.
The distinguishing feature of the prior art device is that interposed between the radiation source and the detector is a magnet arranged on the same side of the material to be gauged. The magnetic field of the magnet is oriented so that by interacting with particles of the ionizing radiation it deflects them arcuately into the sensing area of the detector and disperses them according to their energy levels. This device as compared with single-pass devices is noted for a higher sensitivity in gauging weight per unit area and a higher accuracy as compared with single-pass devices which is due to the double absorption of the radiation by the sheet material and lesser sensitivity to errors. The device is characterized by a wide range of gauging with the use of only one radiation source, since changes in the gauging ranges are effected by placing the detector in the zone where particles of a predetermined energy level are dispersed in dependence on the thickness of the material, but not by replacing the radiation source.
Both a permanent magnet and an electromagnet can be used as the means producing the magnetic field and the size and location of the magnet with respect to the material are selected so that the optimum induction area of the magnetic field is located on one side of the material being gauged.
However, arrangement of the magnetic system between the radiation source and detector according to the prior art concept makes it impossible to efficiently use the activity of the radiation source, since the magnetic field topography in cur instance is such that low-energy particles which comprise a substantial proportion of the ionizing flux, close in the gap between the magnet poles and do not reach the detector. High-energy particles interact with the magnetic field only on a small portion of their path and the magnetic field intensity is not high enough to deflect the radiation particles passing through the material into the sensitive area of the detector, so they scatter in the space. Thus, the detector registers only medium energy particles of those deflected by the magnet. Therefore, the conventional device for safety reasons is provided with a protective shield having a diaphragm adapted to eliminate the medium energy portion of the spectrum. As a result, the required high-energy radiation sources which are utilized to obtain the required accuracy of gauging necessitates a complicated apparatus for affording protection. The problem of a more efficient use of the activity of the radiation source cannot be solved by increasing the intensity of the magnetic field through the increase of the physical size of the magnet located between the radiation source and the detector, because this will lead in this instance to an increase in the distance between the radiation source and the detector, i.e., to an increased length of the air path through which the radiated particles must travel which would cause a still greater scattering of the deflected radiation flux and would thus adversely affect the accuracy of gauging due to changes in such parameters of the surrounding air as temperature, moisture content and pressure. To reduce the dependence of the results of gauging on variations in the parameters of the radiation source and the environment, provision is made for connecting an additional detector, which detector is located in the zone only of the direct radiation flux and not of that passing through the sheet material. But such additional elements complicate the construction of the device, increase its mass and size and impede the efficient use of the source activity.